I used to ask myself this question when I was forced to study grammar in school. Several decades later, I am a professional writer with a strong passion for "proper" English, simply because it makes sense. And I'll tell you why.
Communication is very fragile and misuse of communication leads to misunderstanding. And misunderstanding leads to errors that can have huge consequences, including the saving or loss of life. There must be no misunderstanding when a pilot is landing a commercial jet or a surgeon is performing a delicate operation and much of the reason that these skilled individuals are able to perform their tasks successfully is that everyone around them knows exactly what they are being asked or told to do. And they understand because everyone has agreed to a common set of terms presented in a consistent manner. Without that kind of clear communication there would be chaos (and a lot of lost lives).
In the same way, we must have some common agreement about how we express ourselves in our other activities. It simply allows us to understand each other. The more fully we agree about the meanings of words and how they are presented, the more accurately we can communicate. To do that, there must be rules. Those rules, in this instance, become "proper" English.
But there is some debate about what "proper" English really is. For example, you'll notice that I have started some of my sentences with "And", which most English teachers would tell you is a no-no. However, I use this structure to create a more conversational style of writing. Is it "proper" English? Strictly speaking, no it isn't. But this structure is commonly used in spoken English, which is what I try to emulate in my writing. I (and many others) opt to follow common usage in some areas of my writing. However, that common usage is surrounded by "proper" English and therefore prevents the communication from becoming meaningless gibberish.
One last point: "proper" English is changing all the time. And that means phrases that may not have been deemed "proper" in the past have become acceptable, simply because so many people are now using them. It pains me to think of it, but some day phrases such as "lol", "ttyl" and "brb" may someday be accepted as "proper" English.
Teachers should use appropriate language because he/she doesn't want to hurt the students feeling in case that person will tell his/her parents and the teacher will get suspended.
The new teacher was fired for using explicit language around the students.
The answer to your question depends on society in general using better English. Our English has gotten sloppy and I often cringe when I watch the evening news or read a newspaper. We currently use too many swear words and slang words in speaking and writing. Students learn language from home and friends while most teachers should be role models for good language. As a teacher I refuse to "water down" my language for my students. Language is power and the ability to express your self is important.
Suggestopedia and silent way both involve the teacher building students' confidence in speaking the target language through means other than direct student-teacher conversational exchange. In silent way, the teacher rarely speaks, but in suggestopedia, the teacher recites words and texts to the students.
Students should use respectful and polite language when speaking to teachers in class. This includes addressing them as "Mr./Ms./Mrs." or their appropriate title, using formal language, and listening attentively to their responses.
As a teacher, the woman is dedicated to her students.
The animal was mistreated by the owner. The teacher mistreated the students.
A teacher allows students to write school assignments in their native language.
It is unethical for teachers to make students clean a teacher's house. The teacher's decision to sell grades was unethical.
College students that want to become a teacher are progressive.
The teacher exhibited tolerance of the poorly behaved students.
A teacher using the Socratic method teaches by asking open-ended questions to encourage critical thinking and challenge students' beliefs. Through dialogue and discussion, the teacher helps students explore complex ideas and develop their reasoning skills. The focus is on guiding students to uncover knowledge themselves rather than simply providing answers.
Her teacher told her about the world by using sign language.